This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-353168, filed Dec. 13, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates generally to a magnetic resonance imaging apparatus, and more particularly to a magnetic resonance imaging apparatus for performing 3D imaging using a fast spin echo method.
In modern magnetic resonance imaging apparatuses, an imaging method, which adopts a pulse sequence of a multi-echo method for acquiring multi-echo data with repeated refocusing effected by application of RF pulses, is widely used as an ordinary imaging method for cross-sectional imaging of a subject. In the following description, this imaging method is representatively referred to as a fast spin echo method (FSE method). The fast spin echo method is described in Hennig J, Multiecho imaging sequences with a low flip angles, MJRI, 78:397-407, 1988, and Keifer B, et al., Image acquisition in a second with half Fourier acquisition single shot turbo spin echo, JMRI 1994; 4(P):86.
As is described in Y. Kassai, et al., 3D half-Fourier fast SE for heavy T2-weighted imaging, in xe2x80x9cProceedings, ISMRM, 4th Annual Meetingxe2x80x9d, p. 736, 1996 and Y. Kassai, xe2x80x9cFast ASE and Clinical Application thereofxe2x80x9d, Medical Review No. 69, pp. 28-34, 1998, there is a tendency that 3D imaging using the fast spin echo method is generally applied to medical imaging diagnosis. This technique is called FASE (Fast Asymmetric Spin Echo) technique.
In a modern technical trend in connection with this, it has become possible to decrease an echo train spacing (ETS) which affects an imaging time, and accordingly attention has been paid to imaging of movable objects such as an internal organ (parenchyma) or blood flows, as described in R C Selmeka, et al., HASTE imaging: Description of technique and preliminary results in the abdomen, JMRI, 6:698-699, 1996, and Y. Kassai, et al., 3D Half-Fourier RARE with MTC for Cardiac Imaging, In xe2x80x9cProcessings, ISMRM, 6th Annual Meetingxe2x80x9d, P. 806, 1998.
However, where the echo train spacing (ETS) is to be decreased for 3D imaging using the FSE method, there arises a problem of artifact on an acquired image due to unnecessary FID signals resulting from refocusing flop pulses.
The present invention aims at enhancing an image quality in fast 3D imaging, and the object thereof is to provide a magnetic resonance imaging apparatus capable of effectively spoiling unnecessary signals occurring when image signals are acquired.
In order to achieve the above object, according to an aspect of the invention, there is provided a magnetic resonance imaging apparatus comprising: an RF pulse application unit for applying an RF pulse to an imaging region; a gradient magnetic field application unit for applying a gradient magnetic field to the imaging region; and a control unit for controlling the RF pulse application unit and the gradient magnetic field application unit according to a pulse sequence, the pulse sequence
(a) applying an excitation RF pulse and a selection gradient magnetic field for selecting and exciting a spin within a desired region in a subject,
(b) applying refocusing RF pulses for generating a plurality of echo signals by successively refocusing the excited spin several times,
(c) applying a first phase encoding gradient magnetic field for providing a phase encoding of a different value to each of the plurality of echo signals, and
(d) repeating the steps (a) to (c) while applying a second phase encoding gradient magnetic field with a varying value in a predetermined direction different from a direction of the first phase encoding gradient magnetic field, and
the pulse sequence applying the second phase encoding gradient magnetic field immediately before each of the refocusing RF pulses, and applying a spoiler gradient magnetic field of a predetermined amount in the predetermined direction between each of the refocusing RF pulses and each of the echo signals, and all the gradient magnetic fields applied in the predetermined direction satisfying a phase condition of a CPMG method.
According to another aspect of the invention, there is provided a magnetic resonance imaging apparatus comprising: RF pulse application means for successively applying a plurality of exciting or refocusing RF pulses to an imaging region; and gradient magnetic field application means for applying a plurality of gradient magnetic fields such that all gradient magnetic fields applied in a predetermined direction satisfy a phase condition of a CPMG method and an application amount either immediately after the RF pulse application means has applied the refocusing RF pulses or immediately after a magnetic resonance signal has been read out becomes a predetermined amount or more.
According to still another aspect of the invention, there is provided a magnetic resonance imaging apparatus for acquiring a 3D magnetic resonance image by executing a pulse sequence of a fast spin echo method, wherein two-dimensional image data in first and second directions is acquired within one to several shots, two-dimensional image data in a third direction is acquired by repetition of shots, and the magnetic resonance image is produced by a half-Fourier transform in a direction of shots.
According to still another aspect of the invention, there is provided a magnetic resonance imaging apparatus for acquiring a 3D magnetic resonance image by executing a pulse sequence of a fast spin echo method, the apparatus comprising: RF pulse application means for applying an RF pulse to an imaging region; gradient magnetic field pulse application means for applying a gradient magnetic field pulse of a predetermined amount or more for suppressing an undesired signal occurring due to the RF pulse applied by the RF pulse application means, while performing phase encoding for 3D imaging; acquisition means for acquiring two-dimensional image data in first and second directions within one to several shots and two-dimensional image data in a third direction by repetition of shots by controlling the RF pulse application means and the gradient magnetic field pulse application means, thus acquiring three-dimensional image data; and generation means for generating the magnetic resonance image by applying a half-Fourier transform in a direction of shots to the three-dimensional image data acquired by the acquisition means.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.